The invention relates to the field of intravascular medical devices, and more particularly to an improved balloon for a catheter.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of a dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient's coronary artery until the distal end of the guidewire crosses a lesion or obstruction to be dilated. The dilatation catheter having an inflatable balloon on the distal portion thereof is then advanced into the patient's coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion or obstruction. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g., greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to effect the dilatation without over-expanding the arterial wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow may resume through the dilated artery and the dilatation catheter can be withdrawn from the patient.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, or obstructions that cannot be resolved by inflation of the balloon alone. These conditions often necessitate either another angioplasty procedure, or some other method of repairing, strengthening, or unblocking the dilated area. To reduce the restenosis rate and to strengthen or unblock the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion.
Catheter balloons are typically manufactured independently of the catheter itself and then secured to the catheter with an adhesive or other bonding method. In standard balloon manufacture, a polymer tube is blown biaxially under the action of axial tension, internal pressure, and heat within a mold. The polymer tube may either be simultaneously stretched in the radial and axial directions, or sequentially by first stretching axially and then radially. The starting dimensions of the polymer tube and the finished dimensions of the blow-molded balloon within the mold are a measure of the degree to which the polymeric material has been stretched and oriented during balloon blowing, and affect important characteristics of the finished balloon such as rupture pressure and compliance. The blow-up-ratio (BUR) refers generally to the ratio of the diameter of the blown balloon to the diameter of the undeformed polymer tube. Above a critical BUR for a selected polymer, the balloon blowing process becomes unstable and the polymer tubing will rupture or tear before a balloon is fully formed.
In the standard blow molding process, an initiated air bubble in the polymer tube rapidly expands until the polymer tube forms a balloon that is eventually constrained by the mold wall. The hoop stress in the wall of the tubing, as it grows into a balloon, may be approximated by the expression:σh=(P·R)/δwhere P is the inflation pressure, R is the mean radius of the polymeric tube at any time during the inflation and δ is the wall thickness of the tubing. To form a balloon from the tubing, the inflation pressure P should be such that the wall hoop stress exceeds the material resistance (typically the yield stress) to stretching at the blowing temperature. Once a balloon begins to form from the tubing, it grows rapidly in size until it touches the mold wall. As the balloon expands, its radius R increases and its wall thickness δ decreases. This results in a rapid increase in the wall hoop stress σh during constant pressure blowing. If the wall hoop stress of the growing balloon exceeds the ultimate hoop strength of the material, rupture will occur. This phenomena limits the maximum attainable BUR for a given polymeric material forming the balloon layer.
In the design of catheter balloons, balloon characteristics such as strength, flexibility and compliance must be tailored to provide optimal performance for a particular application. Angioplasty and stent delivery balloons preferably have high strength for inflation at relatively high pressure, and high flexibility and softness for improved ability to track the tortuous anatomy and cross lesions. The balloon compliance is chosen so that the balloon will have the required amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon the application of internal pressure. Noncompliant balloons, for example balloons made from materials such as PET, exhibit relatively little stretching during inflation, and therefore provide controlled radial growth in response to an increase in inflation pressure within the working pressure range. However, noncompliant balloons generally have relatively low flexibility and softness, making it challenging to provide a low compliance balloon with high flexibility and softness for enhanced catheter trackability. A compromise is typically struck between the competing considerations of softness/flexibility and noncompliance, which, as a result, has limited the degree to which the compliance of catheter balloons can be further lowered.
As a balloon is formed by the process described above where an extruded tube is expanded into a mold cavity, the balloon's wall always exhibits a gradient in circumferential orientation of the polymer molecules within. Moreover, the highest degree of orientation occurs at the inner surface and the outer surface experiences the lowest degree of orientation. The gradient is nonlinear, and arises because the percent change in circumference at the inner surface is always greater than that at the outer surface.
There have been attempts to develop methods to raise the overall degree of orientation within the wall of a balloon during expansion. One method employs extruded tubing containing multiple layers of different durometers materials, with the material possessing the highest elongation (typically the lowest durometer) as the innermost layer to enable an increase in the “blow-up ratio” at the balloon's inner surface. Another method utilizes a two-stage expansion process, in which the extruded tubing is first expanded into a mold cavity of intermediate size before being subsequently expanded again into a final, larger mold. This so-called “double-blow” method helps to make the initiation event during balloon expansion less severe and enables the processing of balloons possessing a greater overall BUR value at their innermost surface.
When extruded tubing is expanded circumferentially into a balloon mold, invariably some degree of axial elongation also occurs. The actual degree of axial elongation can be calculated using measured values of initial, “as-extruded” inner diameter (ID) and outer diameter (OD), as well as the final balloon OD, and final balloon double-wall thickness. The calculated value, known as the “area draw-down ratio” (or ADDR) is the ratio of the extrusion's original cross-sectional area to the final balloon's cross-sectional area. The value of ADDR is mathematically equivalent to the ratio of the final length to the original length of the material which comprises the balloon's working length. Higher values of ADDR represent greater axial elongation during the balloon's formation.
The current design and processing approach for balloons purposely involves imparting a substantial amount of axial elongation during balloon expansion. A target ADDR value of 3.0 is common, which means that the material within the wall of the balloon's working length is stretched to 3 times its original length. This amount of axial elongation must be accounted for in the design of the extruded tubing. The extrusion's ID is already made as small as possible by the need to maximize the blow-up ratio, so it is necessary to account for the high degree of axial stretch by increasing the extrusion's OD. Geometric calculations are routinely performed to ensure that the correct final wall thickness will result when a proposed extruded tube is expanded into the desired balloon mold size with an ADDR value of about 3.0. To help control the lot-to-lot variability in axial flow behavior of the extruded tube, a specification exists for the tube's as-extruded elongation to failure (nominally 225% elongation to failure, which translates to an ADDR value of 3.25). Additionally, the amount of externally applied tension may be varied in order to help control the lot-to-lot variability in the axial elongation and thus wall thickness.